Astrobiology Research Priorities for Primitive Asteroids
Effects of asteroid interactions with the biosphere
Download 77.38 Kb.
|
- Navigate this page:
- The Impact Hazard
- Space Missions for the Exploration of Asteroids
- Main-belt Comet Explorer
- Trojan Reconnaissance
Effects of asteroid interactions with the biosphereAsteroids and the Late Heavy BombardmentThe flux of small bodies at Earth during the Hadean eon (4.5-3.8 Gy) is of particular interest because Earth’s differentiation had been completed and its surface cooled. The end of this eon is closely linked to the earliest evidence for habitable conditions on our planet (e.g., Mojzsisi et al., 1996). The epoch around 3.9 Gy ago is particularly crucial because of the growing evidence for a large spike in the bombardment rate, the so-called Late Heavy Bombardment (LHB), of the terrestrial planets (Tera et al. 1974, Cohen et al., 2000, Strom et al. 2005). It is likely that the LHB projectiles were asteroids ejected from the main asteroid belt by means of a large scale dynamical instability. It is thought that this instability was related to the orbital migration of the giant planets (Gomes et al. 2005, Strom et al 2005, Levison et al. 2007). Calculations by Minton & Malhotra (2008) have demonstrated that the migration of Jupiter and Saturn would have caused orbital resonances to “sweep” through the asteroid belt, dynamically launching asteroids into the inner Solar System and greatly depleting the asteroid reservoir, possibly by more than 90%. The orbital migration of the giant planets was fueled by about 30 Earth-masses of outer solar system planetesimals (Hahn & Malhotra 1999, Tsiganis et al. 2005), only ~0.1 Earth-masses of which survive in the present-day Kuiper belt. Thus, the LHB was likely a large spike in the flux of both asteroidal and cometary impactors throughout the Solar system. Outstanding questions about this event include:
The Impact HazardAsteroid impacts have been responsible for mass extinctions in geologic history and remain today the most potentially destructive natural disaster facing humanity. For this reason, it is important to understand the near-Earth asteroid population. Near-Earth asteroids pose a serious and credible threat to humankind. Several such near-Earth objects have only been discovered within days of the objects' closest approach to Earth and recent discoveries of such large objects indicate that many large near-Earth asteroids remain undiscovered. Asteroid collisions rank as one of the most costly natural disasters that can occur. Basic information is needed for technical and policy decision making for the United States to create a comprehensive program in order to be ready to eliminate and mitigate the threats posed by potentially hazardous near-Earth asteroids. The physical processes that determine the transport of asteroids across the Solar System are dominated by the gravitational dynamics of the major planets, particularly the orbital resonances and dynamical chaos that arise from the long term and long range combined effects of several planets. However, non-gravitational forces, such as due to solar radiation, become increasingly important for smaller size sources. The absorption and reemission of sunlight by small asteroids produces a thermal force, known as the Yarkovsky effect that results in a non-gravitational acceleration and produces a slow but steady drift in the semi-major axis of a NEO’s orbit (Chesley et al. 2003; Bottke et al. 2006). Photon emission thus produces a Sun-powered thrust that can move small asteroids by several AU over their history, pushing them into gravitational resonances or into collisions with planets. The Yarkovsky effect must be fully understood before we can predict the long-term dynamics of potentially hazardous objects with a high degree of fidelity. These concerns are addressed by several key questions:
Space Missions for the Exploration of AsteroidsAsteroid Sample ReturnSpacecraft studies of asteroids have greatly improved our knowledge of these bodies. The most important of these is the complete analyses of the data on 433 Eros returned from the Near Earth Asteroid Rendezvous (NEAR) mission and the successful encounter of the Japanese Hayabusa spacecraft with asteroid Itokawa. These encounters demonstrated the diversity of asteroids and their complexity both internally and on the surface. An asteroid sample return mission will provide an enormous scientific payoff. Sample return missions have consistently led to paradigm changing results. The Apollo samples revealed the magma-ocean stage of lunar history and the Giant-Impact Hypothesis for the origin of the Earth and Moon. The Genesis mission measured the chemical and isotopic composition of the Sun. The Stardust mission redefined our understanding of early Solar System dynamics. Asteroid sample return is the logical next step in asteroid science. The highest value sample is pristine carbonaceous material from the early Solar System. In particular such a mission should strive to return material not known to be on Earth. In particular, friable, volatile-rich, and organic-rich material is unlikely to survive passage through the atmosphere and residence on the surface of the Earth. Contamination control and documentation is essential to achieving this objective. Asteroidal samples offer a unique record of the complex chemical evolution that occurred in the early solar nebula. Despite having abundant samples in the form of meteorites, these samples lack geologic context. An asteroid sample return mission would acquire samples with known geologic context As recommended in the NRC report New Opportunities in Solar System Exploration, such a mission should have the following science objectives:
Main-belt Comet ExplorerComets have been discovered in the asteroid belt. Several members of the Themis asteroid family have been observed to display temporary comet-like activity (Hsieh and Jewitt 2006). Seasonal heating effects in are only sufficient to drive sublimation on these bodies for a portion of their orbits, from the time of perihelion to halfway to aphelion (Hsieh 2007). The recurrent activity of these asteroid-comet transition objects is thought to result from seasonal variations that periodically illuminate ice-rich surfaces. Unlike other known comets, these main belt comets appear to have formed in the much warmer inner Solar System, where they are found today. Orbiting completely within the main asteroid belt, the main belt comets present a distinct contrast with other periodic comets, the Jupiter-family and Halley-family comets, which originate in the cold outer solar system in the Kuiper Belt or Oort Cloud. Main belt comets are more accessible than all other comets, making them excellent targets for a spacecraft investigation. A rendezvous mission to a Main Belt comet would provide monitoring of changes in activity. Such a mission should have the following science objectives:
Trojan ReconnaissanceThe Trojans, now known to number well over a thousand, are aggregated about the L4 and L5 equilibrium points along Jupiter’s orbit. These objects are asteroids with a low albedo and a featureless, reddish spectrum. They may have been captured in the present orbits during giant planet formation and migration. Trojan asteroids could prove rich in organics, perhaps sampling regions of the nebula and types of organics not sampled on Earth or by previous missions. Detailed study of these objects should greatly expand our understanding of the history of volatiles and organic molecules in the early Solar System. As recommended in the NRC report New Opportunities in Solar System Exploration, such a mission should have the following science objectives:
Directory: uploads -> filer public uploads -> Ownership Models There are two different types of ownership models uploads -> Police Militarization uploads -> City of rising star regular meting thursday, july 14, 2016 uploads -> Township of winfield uploads -> Stop Militarizing Law Enforcement Act of 2014 uploads -> The Black Panther Party’s Ten-Point Program What We Want Now! filer public -> Midlands State University, Zimbabwe Introduction filer public -> Providing access to online research literature a guide for librarians filer public -> Routes to online research literature : a guide for partners and funders filer public -> Programmes which support access to information 1Access to Global online Research in Agriculture (agora) Download 77.38 Kb. Share with your friends:
|